An image sensor module includes a lens system having a plurality of lenses and an infrared cut-off filter, and an image sensor, in which the infrared cut-off filter functions to cut off an infrared component from an incident light.
FIG. 1 illustrates a lens system 100 provided with an infrared cut-off filter. The lens system 100 includes first to third lenses 110, 120 and 130, and an infrared cut-off filter 140 which are arranged in this order from an object side to an image surface 150 side. For example, the image surface 150 can be a light receiving surface of an image sensor.
Each of the first to third lenses 110, 120 and 130 refracts an incident light, and to form an image of a subject on the image surface 150. The infrared cut-off filter 140 cuts off an infrared component from the incident light. The infrared cut-off filter 140 is implemented by laminating a multi-layered infrared cut-off layer on one side or each side of a glass substrate with opposite flat sides. The infrared cut-off filter 140 is designed in such a manner that infrared lights reflected from the layers of the infrared cut-off layer are destructively interfered with each other to disappear.
FIG. 2 illustrates a transmittance characteristic of the infrared cut-off filter 140. In FIG. 2, the horizontal axis indicates wavelength, and the vertical axis indicates transmittance. FIG. 2 shows a transmittance graph 160 for a white light, of which the incident angle is zero (0) degrees with reference to the normal line of the infrared cut-off filter 140, and a transmittance graph 170 for a white light, of which the incident angle is twenty (20) degrees with reference to the normal line. As illustrated, it will be appreciated that as the incident angle is increased, a cut-off wavelength in the long wavelength side shifts to the short wavelength side. This means that as the incident angle is increased, the red components of lights passing the infrared cut-off filter 140 are reduced. This causes a phenomenon that makes a central area on the light receiving surface of the image sensor appear redder than a peripheral area on the light receiving surface.
As described above, the infrared cut-off filter has a transmittance characteristic sensitive to the incident angle. However, infrared cut-off filters are designed based on a vertically incident light. When such an infrared cut-off filter is applied to an image sensor module, there arises a problem in that the central area on the light receiving surface of the image sensor appears redder than the peripheral area on the light receiving surface. In order to solve this problem, in the process of regulating the white balance of the image sensor, a gain value of the peripheral area of the image sensor is set to be higher than that of the central area of the image sensor in consideration of the relative illumination of the lens system and the transmittance characteristic of the infrared cut-off filter. However, this causes a problem of increasing noise.